Today, irone extracts are produced by solvent extraction or steam distillation of the rhizomes of any iris species, preferably Iris pallida or Iris germanica. After a cultivation period of 2-3 years the rhizomes are harvested and stored for 3-5 years at room temperature. During this maturation period the irones are released from their precursors (W. Krick, F. J. Marner, and L. Jaenicke, Z. Naturforsch. 1983, 38c 179-184). Then the irones are recovered by steam distillation (as iris butter) or solvent extraction (leading to a resinoid) of the powdered rhizomes leading to valuable iris materials for the flavour and fragrance industry.
This long period of maturation is cumbersome, expensive, needs extensive storage capacity and leads to the loss of part of the isomers in uncontrolled degradation reactions.
Several alternatives to this non-effective and time consuming maturation period have been proposed. In a first proposal (FR-2 620 702, 24/3/89) by Buono et al., a lipohilic extract of the iris rhizomes containing the irone precursors is oxidized with KMnO.sub.4 in an organic solvent; in this procedure the extraction of the precursors is laborious and due to this unusual downstream processing (working-up), namely the extraction of the precursors before the oxidation, the quality of the resulting product is quite different from the traditionally preferred Iris Butter or Absolute. A reduction of the maturation period to two months has been disclosed in FR-2 653 637, 3/5/91, by Baccou et al., with a preliminary treatment of the fresh, i.e. not maturated iris rhizomes with ionizing radiation; but the claimed yields hardly reach the irone contents (&lt;400 mg/kg dry rhizomes) obtained during maturation by a 2-3 years period of storage. Bioconversions of lipophilic extracts of the rhizomes with fungi of the genus Botryonia (EP 0 443 925; 23/8/91) by Gil et al. or with peroxidizing enzymes (EP 0 443 926; 28/8/91) by Gil et al., have been disclosed; they lead to a good conversion of the precursors but again present the disadvantage of the preliminary solvent extraction and consequently lead to a product whose quality differs from the preferred iris butter. The bioconversion of powdered iris rhizomes by bacteria of the family Enterobacteriacea in the presence of a plant cell medium (EP 0 353 683, U.S. Pat. No. 4,963,480) by Belcour et al., releases the irones from their precursors, and, after steam distillation produces an iris butter of excellent quality characterized by a high content of irones. A drawback is that the long maturation period has been replaced by a rather expensive microbiological treatment.